1. Field of Invention
This invention relates to the art of injection molding a foamable resin product and, more particularly, to the process and apparatus for heating the cavity of the mold uniformly in order to achieve a smooth surface finish on the part to be produced. In the manufacturing of molded plastic parts, many applications require that the finished part have a glossy surface finish when the part comes out of the mold. In a previous patent issued to the co-inventor, Hendry, U.S. Pat. No. 4,201,742, a method and apparatus for achieving such a finish was presented. This apparatus used a valve means for injecting condensable vapor into the mold cavity in an effort to heat the skin or surface of the cavity above the deformation temperature of the particular resin that was being injected into the mold. Problems of non-uniform heating and leakage through the valve openings caused problems which sometimes gave a non-uniform albeit a smooth finish. The problem was that the internal mold cavity was not being uniformly heated and condensate was collecting in the sprue opening.
An object of this invention is to uniformly heat the working surface of the mold cavity using a condensing vapor injected directly inside the mold but having a slit type orifice for distributing the condensable vapor from a channel or manifold around the periphery of the mold cavity. Another object of the invention is to have a second channel means with a second slit orifice for venting the condensate and excess condensable vapor from the mold just prior to the injection cycle. An object of the invention is to reduce the cycle time of the process.
Another object of the invention is to use a valve means located in one of the mold halves to seal the sprue opening during the condensate cycle of the process and upon operating that valve use that same opening as a sprue gate for injecting the plasticized resin into the mold cavity. It is a further object of this invention to use chilled water for cooling the mold during the cooling cycle. It is further the object of this invention to use steam traps on all of the condensate lines. It is an object of this invention to use condensing vapor for heating the mold surface to a temperature that exceeds the heat distortion point of the material being molded. It is an object of this invention to size the slit orifices so it will pass condensable vapor and condensate but prevent the plasticized resin from expanding into the orifices.